Unicorn that receives USR2 signal on upstart's "stop unicorn", but also allows upstart to respawn it when for some reason it crashed on its own.

an upstart unicorn.conf

# unicorn

description "unicorn ruby app server"

start on (local-filesystems and net-device-up IFACE=lo and runlevel [2345])
stop on runlevel [!2345]

env WORKDIR=/data
env PIDFILE=/data/tmp/pids/unicorn.pid
env CFGFILE=/data/config/unicorn.rb
env CHOWN=deploy:deploy

respawn

script
  trap "start-stop-daemon --signal HUP --stop --pidfile $PIDFILE" HUP
  [ -e $PIDFILE ] && {
    pid=$(cat $PIDFILE)
    while kill -0 $pid &>/dev/null; do sleep 2; done
  } || {
    start-stop-daemon --start --pidfile "$PIDFILE" --chdir "$WORKDIR" --chuid "$CHOWN" --exec "$WORKDIR/bin/unicorn.sh" -- -c "$CFGFILE"
  }
end script

pre-stop script
  [ -e $PIDFILE ] && kill -0 $(cat $PIDFILE) &>/dev/null && {
    start-stop-daemon --signal USR2 --stop --pidfile "$PIDFILE"
    start   # don't send TERM and KILL!
  }
end script

shell output

root@testbox:/etc/init# start unicorn
unicorn start/running, process 2823

root@testbox:/etc/init# pgrep -lf unicorn
2824 unicorn.sh master -c /data/config/unicorn.rb                                                                                               
2953 unicorn.sh worker[0] -c /data/config/unicorn.rb                                                                                            
2956 unicorn.sh worker[1] -c /data/config/unicorn.rb                                                                                            

root@testbox:/etc/init# stop unicorn
unicorn start/pre-stop, process 2823
        pre-stop process 2961

root@testbox:/etc/init# pgrep -lf unicorn
2965 unicorn.sh master -c /data/config/unicorn.rb                                                                                               
3092 unicorn.sh worker[0] -c /data/config/unicorn.rb                                                                                            
3095 unicorn.sh worker[1] -c /data/config/unicorn.rb                                                                                            

root@testbox:/etc/init# initctl list | grep -i unicorn
unicorn start/running, process 2823

root@testbox:/etc/init# kill 2965

root@testbox:/etc/init# pgrep -lf unicorn
3112 unicorn.sh master -c /data/config/unicorn.rb                                                                                               
3241 unicorn.sh worker[0] -c /data/config/unicorn.rb                                                                                            
3244 unicorn.sh worker[1] -c /data/config/unicorn.rb                                                                                            

root@testbox:/etc/init# initctl list | grep -i unicorn
unicorn start/running, process 3111

unicorn.rb

project_folder = File.expand_path("../..", __FILE__)
sockets_folder = File.join project_folder, 'tmp', 'sockets'
pids_folder    = File.join project_folder, 'tmp', 'pids'
logs_folder    = File.join project_folder, 'logs'

#  -- buffer sizes from sysctl net.core.rmem_max=229376 & net.core.wmem_max=229376
#  -- backlog size from sysctl net.core.netdev_max_backlog=1000
listen File.join(sockets_folder, 'unicorn.sock'), backlog: 64, rcvbuf: 131071, sndbuf: 131071

preload_app false
worker_processes 2
timeout 30

# chdir into this folder on restarts, fixes capistrano's `current` symlink to be workdir
working_directory project_folder

stderr_path       File.join(logs_folder, 'unicorn_error.log')
stdout_path       File.join(logs_folder, 'unicorn_output.log')
pid               File.join(pids_folder, 'unicorn.pid')

# change who is the unicorn executable - so it can reload itself even when rvm ruby was changed
# this will be the "new" executable unicorn loads when it receives a USR2 signal.
Unicorn::HttpServer::START_CTX[0] = File.join(project_folder, 'bin', 'unicorn.sh')

before_exec do |server|
  # Bundler loads gem environment from original deploy path instead of current
  # Capistrano's `current` symlink will break deploys without this tweak.
  ENV["BUNDLE_GEMFILE"] = File.join(project_folder, 'Gemfile')
end

before_fork do |server, worker|
  old_pid = server.config[:pid] + '.oldbin'
  if File.exists?(old_pid) && server.pid != old_pid
    begin
      Process.kill("QUIT", File.read(old_pid).to_i)
    rescue Errno::ENOENT, Errno::ESRCH
      # someone else already killed old master
    end
  end
end

unicorn.sh

#!/bin/bash
set -e

source /usr/local/rvm/scripts/rvm
rvm use 1.9.3 &> /dev/null
exec bundle exec unicorn "$@"

@kesor thanks for this, this is the most reasonable solution to accomplish zero downtime w/ unicorn for us. We added forced respawn and a respawn limit and had to wire up pre-up stuff to dial it in a little more so it would fire up on deployment and restart properly, but it's beautifully seamless.

respawn
respawn limit 10 10

pre-start script
  if [ -e $PIDFILE ] && [ ! -e /proc/$(cat $PIDFILE) ]; then
    rm -f $PIDFILE
  fi
end script